Sulphide deposits have dominated production of primary zinc. The sulphides are readily concentrated by flotation and the concentrate is either smelted or undergoes a roast-acid leach-electrowinning process to recover zinc. Oxide deposits are not readily concentrated. However, increasing zinc demand has resulted in increased interest in processing oxidised zinc ores. In one instance of which the inventors are aware, a zinc silicate ore has been subjected to an acid leach, followed by solvent extraction and electrowinning.
However, acid leaching is non-selective, with many elements dissolving in addition to the target metal, most notably iron and silicon. These elements have to be removed subsequently, increasing substantially the complexity of the process. Furthermore, additionally, some silica-containing zinc ores have an additional complication. For example, willemite (Zn2SiO4) has been leached with acid, but tends to give a silica gel making solid-liquid separation difficult, similar problems have been reported for hemimorphite (Zn4Si2O7(OH)2. H2O) (S. Espiari, F. Rashchi, S. K. Sadrnezhaad, Hydrometallurgical treatment of tailings with high zinc content; Hydrometallurgy, 2006, 82 (1-2) 54-62).
The process at the Skorpion Mine, Rosh Pinah, Namibia has been previously described in detail (J. Gnoinski, Skorpion Zinc: optimization and innovation, The Journal of The Southern African Institute of Mining and Metallurgy 2007, 107(10) 657-662) and uses carefully controlled conditions to prevent gel formation. The leaching at Skorpion takes place at 50-60° C. which, although it enhances the solubility of silica, results in formation of a filterable silica precipitate when the pH is raised using lime (CaO), iron and aluminium are also precipitated during this step. The solid precipitate has then to be separated from the pregnant solution via thickening. The raised temperature, necessity to precipitate impurities and then perform solid-liquid separation all increase the complexity of the flowsheet and both capital and operating costs.
The removal of impurities from acid solutions can be achieved either by precipitation followed by cementation (as practiced by the roast-leach-electrowin plants) or by solvent extraction (as practiced at Skorpion). Further technical details of the precipitation and cementation stages of the roast-leach-electrowin process can be found in “The Extractive Metallurgy of Zinc” by R. J. Sinclair, AusIMM, 2005.
The complexity of the solvent extraction process at Skorpion is clear with a total of eight consecutive stages required to achieve the desired purity. (K. C. Sole, A. M. Feather and P. M. Cole, Solvent extraction in southern Africa: An update of some recent hydrometallurgical developments; Hydrometallurgy, 78 (1-2) (2005) 52-78).
Additionally, in the presence of oxide, carbonate or hydroxide mineralogy (zincite ZnO, hydrozincite 2ZnCO3.3Zn(OH)2 and smithsonite ZnCO3) the acid consumption may also be extremely high making it uneconomic to process the ore using acid.
The method of leaching of the present invention has as one object thereof to at least partially overcome the abovementioned problems associated with the prior art, or to at least provide a commercially useful alternative thereto.
The preceding discussion of the background art is intended to facilitate an understanding of the present invention only. The discussion is not an acknowledgement or admission that any of the material referred to is or was part of the common general knowledge as at the priority date of the application.
Throughout this specification, unless the context requires otherwise, the word “comprise”, or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.
Throughout this specification, unless the context requires otherwise, the term “ore” or variations thereof, will be understood to include, for example, the product of one or more pre-treatment steps, such as a roast or calcination steps, or one or more concentration steps, but is not limited thereto.